Polyimide (PI) is known to have excellent thermal stability, low dielectric constant, and high resistance against chemical corrosion. Because of these advantageous properties, polyimide has been widely used in the electronic industry, most notably as a base material, or the so-called substrate, in electronic packaging applications, such as in the manufacturing of tape automated bonding (TAB), flexible printed circuit board (FPC), multichip modules (MCM), etc.
However, in the commercial applications of polyimide, particularly in the areas related to tape automated bonding and flexible circuit board, polyimide lacks the required adhesion to be used alone as a substrate. As a consequence, an adhesive layer is often required between the copper foil and the polyimide substrate to bond the same together. FIG. 1 shows a flowchart diagram describing the process in making the polyimide/adhesive/copper foil laminate.
Because of the extra step required and the relative complexity involved in the process of making the polyimide-/adhesive/copper foil laminate, the cost of manufacturing therefor has become a significant portion in the total cost of making tape automated bonding or flexible printed circuit board. Furthermore, the adhesive layer often does not have the same heat resistance and/or weatherability as the polyimide substrate, thus it often compromise the quality of the polyimide-based substrate. Typical adhesives that have been used in making the polyimide-/adhesive/copper foil laminate include acrylic and epoxy resins. These resins are known to lack thermal resistance and often became bursted during the welding of the laminates.
European patent application EP 0 383 461 A2 discloses a process for producing a flexible printed base by directly coating a copper foil with a polyimide precursor, followed by heating, drying and curing the precursor. The polyimide precursor has repeating units of aromatic tetracarboxylic acid anhydrides, such as 2,3,3',4'-tetracarboxydiphenyl, and aromatic diamines, such as diaminosiloxanediamine. Although the siloxane compound improves the adhesion between the polyimide substrate and the copper foil, it also introduces other problems, such as the lowering of the glass transition temperature, Tg, of the imide copolymer, and the heat resistance thereof. Also, because of some compatibility problems that exist between siloxane and polyimide molecules, phase separations often occur as a result, thus causing adverse effects on the appearance of the final product and the physical properties thereof.
U.S. Pat. No. 3,247,165 (the '165 patent) discloses a polyimide composition formed by the reaction of benzophenone tetracarboxylic acid dianhydride and certain heterocyclic diamines. Examples of heterocyclic diamines include benzimidazole, benzoxazole, benzothiazole, etc. The object of the polyimide compositions disclosed in the '165 patent was to provide improved heat resistance; they are not suitable for use as a base material in the production of polyimide/copper foil laminates to be used in FPC or TAB applications.